To date, ethylene which is more than 99.8% pure is usually used for chemical synthesis. This ethylene of very high purity is obtained via the cracking of various petroleum products, followed by numerous complex and expensive separation operations in order to isolate the ethylene from the other products of cracking and to obtain a product of very high purity.
In conventional ethylene plants, as described for example in the patent EP 0872537 B1, the crude gas compression is followed by a complicated separation process, in which ethylene and propylene of high purity (polymer quality) are produced. Further products may be butadiene, benzene, toluene, styrene and others. The separation process is characterized by the following steps:
1. Splitting in C2 minus fraction/C3 plus fraction
2. Splitting of the C2 minus fraction:
                2.1. Separation of hydrogen and methane in a deep cooling at approximately −150° C.        2.2. Separation of a C2 fraction        2.3. Splitting of the C2 fraction in ethylene and ethane        2.4. Recycling of ethane to cracking3. Splitting of C3 plus fraction into a C3 and a C4 plus fraction:        3.1 Hydrogenation of propyne and propadiene in the C3 fraction        3.2. Distillation of pure propylene from the C3 fraction        3.3. Splitting of the C4/C5 fraction, depending upon the requirements.        
Integrated into the plant for production of ethylene are refrigeration units to generate temperatures down to −150° C. Pure ethylene and pure propylene serve as refrigerating medium.
For the chemical use of ethylene, a high concentration of >98% is in many cases not necessary.
The patent application WO 03/048088 describes the production of low-concentration ethylene for the chemical reaction with chlorine by means of ethane dehydrogenation. The ethane-loaded gas stream contains not only hydrogen and methane, but also high amounts of unconverted ethane. For the economic design of the process, the unconverted ethane must be fed back to ethane dehydrogenation after complicated cleaning processes. This process can only use ethane as feedstock. A significant disadvantage is the very low concentration of ethylene—less than 60%—as well as the fact that further components of the gas stream such as hydrogen, propylene, butadiene only allow to use the ethylene in very special processes.
In the patent application WO 2006/067188, the production of vinyl chloride is described, starting with the cracking of ethane/liquefied gas as feedstock. The feedstock undergoes the usual cracking. After quenching and water washing, the cracked gas is compressed and cleaned from hydrogen sulfide, carbon dioxide and water, before it is split into three fractions. Fraction A contains ethylene, ethane, methane, hydrogen as well as small amounts of carbon monoxide. Fraction B contains mainly ethylene and ethane as well as small amounts of methane and very low amounts of hydrogen. Both fractions are used for the reaction with chlorine in various processes. A fraction C contains ethane and hydrocarbons with more than 3 carbon atoms. For the separation of a gas mixture, various circuits of at least two columns are described.
For the use of fraction C, various variants are suggested, such as combustion, recycling as feedstock without further treatment or recycling to the feedstock after hydrogenation of unsaturated components contained in fraction C. In accordance with our experiences, the recycling to feedstock without further treatment is only a hypothetic variant, since feedstock with high olefin contents leads to strong coke formation in the pyrolysis furnaces and gives a very low ethylene yield.
Prior to the hydrogenation of fraction C, the splitting into fractions with less or more than 5 carbon atoms is described. Only the fraction with 5 and less carbon atoms undergoes hydrogenation. The description does not contain information on the technology of hydrogenation.
The hydrogenation of propylene is specified in patent application DE 1518827. This patent application describes the hydrogenation of propylene into propane to be returned as feedstock to the cracking furnaces for ethylene production. Hydrogenation takes place in a reactor in a hydrogen atmosphere in the liquid phase (trickle phase reactor). Such reactors and the utilized catalysts based on metals of the eighth subgroup (palladium, platinum) have proven themselves internationally for the hydrogenation of liquid unsaturated hydrocarbons. It is explicitly said that the cracking of unsaturated hydrocarbons leads to increased coke formation.